We have shown that red cell membrane proteins are synthesized at different times in the maturation cycle of the red cell. It is thus clear that the membrane of the mature red cell is not synthesized as a unit. The rabbit reticulocyte is still capable of synthesizing some but not all of its membrane proteins in vitro. We plan to study the biosynthesis of these membrane proteins in a cell-free, reticulocyte lysate system with special emphasis on determining the factors responsible for incorporation of the newly synthesized proteins into the existing membrane structure. The basic question to be answered is whether the primary structure of the protein is the sole determinant of its incorporation into the membane as opposed to soluble proteins such as hemoglobin. We plan also to study the synthesis of membrane proteins by red cell precursors in the marrow, since it apears that certain proteins of great interest are made only at this earlier stage of maturation.